Microscopic Characterization of Polycrystalline APCVD CdTe Thin Film PV Devices

Atmospheric pressure chemical vapor deposition (APCVD) is being studied as an alternative for large-area manufacturing of CdTe thin films. High efficiency research cells have been constructed, but the fundamental materials properties and limitations have not been fully explored. APCVD material is examined with several techniques and compared with close-space sublimation (CSS). Transmission and scanning electron microscopy studies show a similar morphology to CSS CdTe. However high resolution TEM scans show the formation of a disordered layer between the CdTe and CdS, and the removal of defects within some grain structures upon annealing. Cathodoluminescence shows electronic defect states localized to grain boundaries. A large concentration of trap states was also observed with deep-level transient spectroscopy that may correspond to hole traps found in lower amounts in other materials. The presence of traps was also indicated in impedance spectroscopy measurements. The latter studies indicate a high grain boundary resistance contributes to transport.